Method of removing monomeric components from polycaprolactam



H. LUDEWIG Aug. 8, 1967 METHOD OF REMOVING MONOMERIC COMPONENTS FROMPOLYCAPROLACTAM 2 Sheets-Sheet 1 Filed Sept. 11, 1963 nuooooonodasINVENTOR. HERMANN LUDEWIG 1967 H. LUDEWIG 3,335,115

METHOD OF REMOVING MONOMERIC COMPONENTS FROM POLYCAPROLACTAM Filed Sept.11, 1963 2 Sheets-Sheet 2 INVENTOR. HERMANN LUDEWIG United States Patent3,335,115 METHOD OF REMOVING MONOMERIC COMPO- NENTS FROM POLYCAPROLACTAMHermann Ludewig, Rudolstadt, Germany, assignor to VEB ChemiefaserwerkSchwarza Wilhelm Pieck, Rudolstadt, Germany Filed Sept. 11, 1963, Ser.No. 309,915 5 Claims. (Cl. 260-78) The present invention relates to aprocess for making products, such as threads, wires, films and the like,from synthetic linear high polymers, especially from polyamides; moreparticularly, the process relates to making products which are poor inmonomers.

The invention also relates to an apparatus for making such polymerproducts which are poor in monomers.

At present, there are mainly two processes in use for making polyamides,namely (1) the classical grid spinning process, and (2) the directspinning process.

The grid spinning process comprises subjecting polyamide chips preparedin a polymerization apparatus to a number of operational steps, such aspolymerization, comminution, extraction, and drying. The extracted chipsare then melted on a grid, and the melt is formed into threads or othershaped products.

In the direct spinning process, the molten polyamide is directly fedfrom the polymerization apparatus to the spinning head over distributionlines. When in the direct spinning process no additional unit is used,the end product will contain low polymer products in accordance with thechemical equilibrium. For lowering the amount of low polymers in thedirect spinning, the following methods are known:

(a) Vacuum stage: in this, the monomeric components are eliminated fromthe melt by high-vacuum (thin layer evaporation).

(b) Vapor spinning process: This is based on blowing a current ofsuper-heated steam through the polymer melt, said current carrying alongall constituents which are undesirable for the spinning operation.

The known processes and the apparatus for carrying them out have certaindrawbacks in the production and processing of polyamides. In the gridspinning process, complicated equipment entailing high energyconsumption is necessary for lowering the monomeric portions.Furthermore, there is the risk of quality deterioration if polyamide isbrought into contact with the atmosphere.

Material obtained by direct spinning has the disadvantage of highermonomer contents which causes difficulties in the processing industry.The use of the vacuum stage for lowering the monomer content requiresagain complicated apparatus and high energy consumption. The streamspinning process is still under development and there are no reports onthe process when used for largescale production.

It is the object of the present invention to provide a method for makingshaped articles from high polymers, e.g. polyamides, which are free ofthe above-mentioned shortcomings. It is another object to provide amethod whereby the monomeric, i.e. water soluble portions of thepolyamide, are removed in a simple and effective manner.

It is yet another object to provide an apparatus for making the productsabove-defined.

Other objects and advantages of the present invention will becomeapparent from the following detailed description and the accompanyingdrawings.

It has now been found unexpectedly that the abovementioned drawbacks canbe avoided and the amount of water soluble polymer can be decreased,when the molten polymer, especially polyamide, is treated in anautoclave as a eutectic liquid mixture of polyamide/water or poly-Patented Aug. 8, 1967 amide/organic solvent with application of pressureand elevated temperature.

Thus, according to the invention, the polyamide melt is extracted underpressure in an autoclave partly filled with a solvent, e.g. water, as aeutectic liquid mixture of polyamide and solvent, the extraction takingplace above the boiling point of the pure solvent and below the meltingpoint of the pure polyamide; thereby, the soluble (monomeric) portion ofthe polyamide passes into the solvent which is thereafter removed fromthe eutectic melt, whereupon spinning takes place in a known manner.

The extraction of the water-soluble portion takes place in an autoclaveor a pressure tube under the excess pressure of the solvent or thewater. If e-aminocaprolactam is extracted, temperatures will preferablybe used in the range of 160 to 180 C. and pressures of about 10atmospheres excess pressure.

Apart from water, solvents to be used are, e.g. alcoholwater,ketone-water or ketones, and other organic solvents.

vIn some cases, simple stirring of the liquid mixture in water at 160 to180 C. will be sufficient in order to accelerate extraction andaccomplish a thorough elimination of the soluble constituents. Onedesirable method of operation is to feed the polyamide melt to beextracted under pressure and in the shape of threads or thin sheets intothe hot water or other solvent contained within autoclave.

Another method of extracting according to the invention consists inconveying the polyamide melt to be extracted in a thin layer through thehot water or solvent in the autoclave over the guide plates, guidefunnels, guide screens, screen inserts or other similar devices.

The extraction may, e.g. be carried out as described below. The polymermelt, e.g. polyamide melt, prepared in a know manner in an autoclave ora tube for fully automatic polymerization of caprolactam is fed by meansof a pump or a system of pumps into a container filled with a solvent,e.g. water, maintained at the temperature of about 170 C., and an excesspressure of about 10 atmospheres. The water-soluble portions collect inthe water; the liquified polyamide settles at the bottom of the tube andis later, after leaving the container, worked up directly in theeutectic mixture or separated from said mixture, and allowed to solidifyin a known manner, re-

' melted, and shaped. In the manner above-described the process may becarried out as a continuous process.

An advantageous embodiment for further processing of the extracted meltconsists therein that from the eutectic mixture of polyamide andremaining solvent, the latter is removed in a heated intermediate vesselin a manner known in the polymerization process, namely by degassing,whereafter, the polyamide melt is shaped. The removal of solvent can beenhanced by passing through the melt an inert gas, such as nitrogen,carbon dioxide; and the like, or steam.

In accordance with the invention the melt of the polyamid-solventmixture is passed into a vessel heated .up to 250 C. and provided with avalve. The solvent con.- tained in the melt escapes over the valve intothe open, and the melt free of solvent is spun after degassing in theusual manner. Instead of the vessel, a simple or multiple-stage tube canbe used for the degassing. It'is furthermore possible to heat solidpolyamide portions together with water or an organic solvent in theautoclave until it liquefies, but not to a point at whichdepolymerization would occur. Subsequent processing is conventional.

An apparatus for carrying out the process according to the invention isillustrated in the accompanying drawings by way of example. In thedrawings: 1

FIG. 1 shows one embodiment of the apparatus in a schematic side view,partly in section, and

FIG. 2 is a similar showing of another embodiment of theapparatus.

Referring now to FIG. 1, designates a U-shaped tube for fully automaticpolymerization of a polyamide, known in the art as VK-tube. An inletopening for admission of lactam is indicated at 7, an outlet opening forescape of steam at 8. The two limbs of the U-tube are surrounded byheating jackets 11 provided with two short pipes 11a used for servicing.

Tube 10 is connected by means of a pumping system schematicallyindicated at 12 with an elongated vessel 13 in which the extraction isto be performed. Vessel 13 is surrounded by a heating jacket 13a and ispartly filled with a solvent, e.g. water which enters at 9a. A shortpipe 11b is provided at the bottom of the jacket 13a. From the top ofthe tube 1.3, an escape valve 9 for solvent, e.g. water leads outside.

A pluarlity of guide plates 14 is arranged throughout the length of theliquid-filled portion of the tube 13 and extending into a sump portion15 where the mixture of solvent and melt poor in monomers formed in thetube is allowed to collect. The solution above the sump 15 containingthe monomers is continuously replaced with fresh water by means ofpumps.

Joined to the bottom of tube 13 is a conveyor device with a worm 16 fordelivering the liquid mixture of polyamide and remaining water orpolyamide and remaining solvent out of the sump to a degassing vessel 17furnished with a heating jacket 17a and a servicing pipe 11c, from whereremaining water or solvent is allowed to escape as steam or vaporthrough a valve 18, while solvent-free melt collects at the bottom. Aspinning device 19 is directly connected to the vessel 17, and the meltis passed through a plurality of spinnerets 19a.

Instead of the spinning device, another shaping device can be provided,when it is desired to produce e.g. film or other sheet material byextrusion of the hot melt.

In the embodiment of the device shown in FIG. 2, 20 designates anautoclave for the polymerization of caprolactam. Inlet and outletopenings at the top are designated by 31 and 32, a heating jacket by 30aand a short pipe at the bottom of the jacket by 28a. The apparatus isdesigned for working in batches. A valve 21 connects the funnel-shapedbottom of the autoclave 20 with an extraction vessel 23 over line 29.Mounted within the vessel 23 is a stirrer 22 operable by known means(not shown). An escape valve 33 is provided for solvent discharge.

The vessel 23 is surrounded by a heating jacket 30]) which has at thebottom a short servicing pipe 28b.

At the bottom, vessel 23 is connected by a pumping system, schematicallyillustrated at 24, with a degassing vessel 25. This vessel is equippedwith a gas escape valve 34, heating jacket 30c and a servicing pipe 280.Further attached to vessel 25 is a spinning device 27 comprising aplurality of spinnerets for producing filaments or wires 26.

The extraction process according to the invention makes it possible todecrease the monomeric contents of a polyamide containing about 10%thereof down to 1% of monomers and even less.

In the following, the invention will be more fully described in a numberof examples, but it should be understood that these are given by way ofillustration and not of limitation, and that many modifications may bemade without departing from the spirit of the invention.

Example 1 This example of the process is described With reference toFIG. 1.

A melt of caprolactam containing 2% water and acetic acid, namely l/600mol acetic acid per mol lactam, is

continuously passed through the tube 10. The melt of polyamide whichbecomes very highly viscous by polymerization, and which contains 10% ofwater soluble products, is conveyed directly from tube 10 into theelongated extraction or pressure tube 13 by means of the pumping system12. The pressure tube is filled to /3 with water of C. at an excesspressure of 10 atmospheres. The threads pass through the water at 170 C.over guide plates 14 and collected in the sump 15. While passing throughthe water at the temperature of 170 C. the content of the water solubleamount decreases from 10% to 0.5%.

From the sump, the melt of polyamide and remaining water is delivered byWorm 16 into the vessel 17 heated up to 250 C., maintained under normalpressure and nitrogen atmosphere. In the vessel degassing takes place bymaking the water from the melt pass out as steam through the valve 18into open air. After a degassing period of 15 minutes, the Water-freemelt is continuously delivered from the degassing vessel to thespinnerets 19a and is then spun. The threads contain 3 to 4% by weightor less of extract, mainly monomeric components.

The extract-containing water which stands above the sump is continuouslyreplaced by fresh preheated water in a countercurrent stream.

Example 2 This example of the process according to the invention isdescribed with reference to FIG. 2.

A polyamide melt made in a conventional manner from caprolactam in theautoclave 20 is supplied to the extraction vessel 23 in a batch throughopen valve 21 and line 29.- The extraction vessel is about half filledwith hot p-cymol (1-methyl-4-isopropyl-benzene) which has a boilingpoint of 177 C. at normal pressure. The valve 21 is then closed, and thetemperature raised to about 210 C., where it is maintained for about onehour while vigorous stirring is carried out. Thereafter, the stirring isdiscontinued and separation of the polyamide and the hydrocarbon solventeffected by allowing the mixture to settle. The pumping system 24 thendelivers the melt to the degassing vessel 25 maintained at 260 C. wherehydrocarbon vapor is allowed to escape through valve 34. After a shortheating period, the melt now containing only 2% by weight of monomers oroligomers, is passed on to the spinnerets through which spinning occursin the conventional manner.

It should be understood, of course, that the foregoing disclosurerelates only to preferred embodiments of the invention and that it isintended to cover all changes and modifications of the examplesdescribed which do not constitute departures from the spirit and scopeof the invention as set forth in the appended claims.

By shaped structures it is intended to designate filaments, wires,films, sheets and similar articles made from synthetic high polymers, byspinning and extruding, respectively.

What is claimed is:

1. Method of removing monomeric components from a polycaprolactamcontaining the same, which comprises contacting said polycaprolactamwith a solvent for said monomeric components which solvent at normalpressure has a boiling point below the melting point of saidpolycaprolactam and which forms a eutectic mixture therewith whilemaintaining a temperature above the boiling point of the pure solventand below the melting point of the pure polycaprolactam and as high asthe eutectic temperature of said mixture and a pressure which issufiiciently high so that said solvent is in liquid condition at saidtemperature, whereby said polycaprolactam is subjected to liquid-liquidextraction by said solvent although at normal pressure said solventwould be in gaseous condition and said polycaprolactam would be in solidcondition, so that the major portion of said monomeric components aredissolved by said solvent; and

separating said solvent from said mixture, thereby obtaining apolycaprolactam of low monomeric components content.

2. Method according to claim 1 wherein said solvent is selected from thegroup consisting of water, alcohol, ketones, alcohol-Water, ketone-Waterand cymol.

3. Method according to claim 1 wherein said solvent is water and saidtemperature is between about 160 C. and 180 C.

4. Method according to claim 1 wherein said solvent is separated fromsaid mixture by degassing.

5. Method according to claim 4 wherein an inert gas is passed throughsaid mixture during degassing.

References Cited UNITED STATES PATENTS 2,361,717 10/1944 Taylor 260782,731,081 1/1956 Mayner 26078 6 1/1959 Ludewig 26078 10/1961 Upton etal. 188 1/1962 Kjellmark 26078 4/1962 Monroe 2 60-78 7/1962 Ryffel 260787/1962 Braun et a1. 26078 11/1963 Beck 18-8 9/ 1964 Cerutti 260-78FOREIGN PATENTS 10/ 1962 Canada.

Examiners.

L. S. SQUIRES, H. D. ANDERSON,

Assistant Examiners.

1. METHOD OF REMOVING MONOMERIC COMPONENTS FROM A POLYCAPROLACTAN CONTAINING THE SAME, WHICH COMPRISES CONTACTING SAID POLYCAPROLACTAN WITH A SOLVENT FOR SAID MONOMERIC COMPONENTS WHICH SOLVENT AT NORMAL PRESSURE HAS A BOILING POINT BELOW THE MELTING POINT OF SAID POLYCAPROLACTAM AND WHICH FORMS A EUTECTIC MIXTURE THEREWITH WHILE MAINTAINING A TEMPERATURE ABOVE THE BOILING POINT OF THE PURE SOLVENT AND BELOW THE MELTING POINT OF THE PURE POLYCAPROLACTAM AND AS HIGH AS THE EUTECTIC TEMPERATURE OF SAID MIXTURE AND A PRESSURE WHICH IS SUFFICIENTLY HIGH SO THAT SAID SOLVENT IS IN LIQUID CONDITION AT SAID TEMPERATURE, WHEREBY SAID POLYCAPROLACTAM IS SUBJECTED TO LIQUID-LIQUID EXTRACTION BY SAID SOLVENT ALTHROUGH AT NORMAL PRESSURE SAID SOLVENT WOULD BE IN GASEOUS CONDITION AND SAID POLYCAPROLACTAN WOULD BE IN SOLID CONDITION, SO THAT THE MAJOR PORTION OF SAID MONOMERIC COMPONENTS ARE DISSOLVED BY SAID SOLVENT; AND SEPARATING SAID SOLVENT FROM SAID MIXTURE, THEREBY OBTAINING A POLYCARPROLACTAM OF LOW MONOMERIC COMPONENTS CONTENT. 